PGF2α analogues have been widely used for the treatment of glaucoma and ocular hypertension because of their effectiveness in lowering intraocular pressure and their low systemic side effects. PGF2α analogues include all of the known PGF2α analogues for example tafluprost, latanoprost, isopropyl unoprostone, travoprost, bimatoprost and the analogues shown in U.S. Pat. No. 5,886,035, U.S. Pat. No. 5,807,892, U.S. Pat. No. 6,096,783.
Tafluprost is a new-generation, fluorinated PGF2α isopropyl ester analogue, which is a potent ocular hypotensive agent (EP 0 850 926).
A concentration of PGF2α analogues used for a treatment of glaucoma is very low. For example, the effective concentration of tafluprost from 0.0005 to 0.005 (w/v), preferably about 0.0015%, in an ophthalmic composition has been found to be sufficient for the treatment of ocular hypertension and glaucoma. However, as lipophilic substances PGF2α analogues such as tafluprost are liable to be absorbed to resinous (plastic) containers or bottles commonly used to store ophthalmic solutions, and thus the already low drug concentration in the ophthalmic solution may be further lowered.
Preservatives
Preservatives which exhibit sufficient antimicrobial effect on bacteria and fungi have traditionally been used in ophthalmic compositions. In addition to this, the preservatives are required to be stable and preferably to homogenize and stabilize the composition by interacting with the ingredients, for example by homogeneously dispersing or dissolving the ingredients into the vehicle or base (see EP 0 969 846, EP 1 916 002 and EP 1 547 599). Nowadays the most commonly used preservatives in commercially available ophthalmic solutions are benzalkonium chloride (BAK) and other quaternary ammonium salts. Other pharmaceutically acceptable preservatives for ophthalmic solutions are for example boric acid-polyol-zinc chloride (EP 1 115 406) or chlorine oxide compounds (EP 1 905 453), chlorhexidine gluconate, benzethonium chloride, sorbic acid, potassium sorbate, ethyl p-hydroxybenzoate and butyl p-hydroxybenzoate.
However, preservatives are also known as the major etiology of keratoconjunctive disorders, and for safety purpose, it is preferred that the concentration of a preservative such as benzalkonium chloride (BAK) is as low as possible. Preservative free ophthalmic solutions have also been developed.
On the other hand, BAK has contributed to the prevention of the degradation of prostaglandins and to the inhibition of absorption of prostaglandins to the resinous container walls. The absorption of tafluprost and other PGF2α analogues to the resinous container walls has been a problem especially with containers made of polyethylene. Due to its properties, such as sufficient flexibility, softness, good manufacturability and user-friendliness, polyethylene is the preferred material of choice for packaging of ophthalmic compositions, especially in unit dose form.
In addition, absorption of PGF2α analogues to the resinous container walls depends on surface area of the container walls. A unit dose preparation contains very small amount of ophthalmic compositions and the contact area of the preparation to the container is very large. Thus, absorption of PGF2α analogues to the container walls is a severe problem for unit dose preparations.
Therefore, before the present invention it has in practice been impossible to prepare stable, preservative-free ophthalmic solutions which contain PGF2α analogues and which can be packaged and stored in containers consisting essentially of polyethylene. According to EP 1011728, aqueous prostaglandin compositions packaged in polypropylene containers are more stable than those packaged in polyethylene containers. Based on the stability results of said publication, a skilled person is not encouraged to choose polyethylene but is likely to disallow it as container material, especially for all highly lipophilic compounds such as PGF2α analogues. Polypropylene-polyethylene copolymers comprising polyethylene as a minor component are also possible but not alluding to polyethylene as the sole material or as a major resin component (EP 1 829 545).
Furthermore, almost all of PGF2α analogues are practically insoluble in water. It is thus necessary to solve also the problem of solubility to water in order to formulate PGF2α analogues in ophthalmic solutions, especially for unit dose preparations. In EP 1 321 144 and US 2007/248697, a nonionic surfactant has been added to the ophthalmic solution to prevent a prostaglandin derivative from being adsorbed to a resinous container. Other attempts to compensate the difficulties in formulating highly lipophilic prostaglandin analogues in water have been described for example in EP 0 969 846, EP 1 666 043, EP 1 011 728 and WO 2007/042262 but they do not mention any preservative-free composition.
It is therefore an object of the present invention to provide an ophthalmic aqueous solution comprising PGF2α analogues and substantially no preservatives wherein the absorption of PGF2α analogues to the resinous containers consisting essentially of polyethylene is prevented and wherein said analogues remain soluble, stable and bioavailable in a preservative-free preparation. The aqueous ophthalmic solution according to the invention provides a significant clinical advantage as there is currently an unmet clinical need of preservative-free prostaglandin eye drops for glaucoma patients.